Situational irony is a relationship of contrast between what an audience is
led to expect during a particular situation within the unfolding of a story's
plot and a situation that ends up actually resulting later on.

It is thus the result of a special sort of discrepancy in perspective
that is not "moment-bound," in that it involves the contrast
between what we knew in one moment with what we have come to know in
another.

Of course, like dramatic irony, situational irony can range from the tragic
to the comic. In comedy, for example, the surprising reversal in circumstances
making for situational irony portends can be for the better. A classic instance is the
climactic moment in Molière's Tartuffe,
in which the villain Tartuffe, having conned his benefactor Orgon into putting
the title to his property into Tartuffe's name, brings an Officer to Orgon's
house to execute an order of eviction upon the family -- but ends up (to
the surprise not only of everyone present but of the audience as well)
being arrested and dragged off to jail as a crook whom the King, in reviewing
the cases coming before the royal courts, has recognized from his past record of
criminal activity.

But some of the most famous and powerful uses of situational irony are associated with tragedy, where it serves to emphasize
how uncertain human prosperity, and how fragile human happiness, can be.